This invention relates to a magnetic core of an inductance device such as a choke coil, transformer or the like, particularly, relates to a magnetic core (which will hereinunder be often referred to as xe2x80x9ccorexe2x80x9d simply) which has a permanent magnet as a magnetically biasing magnet.
To a choke coil and a transformer used in, for example, a switching power supply or the like, an AC current is usually applied thereto together with a DC current superposed thereto. Therefore, a core used in those choke coil and transformer is required to have a magnetic characteristic of a good magnetic permeability so that the core is not magnetically saturated by the superposition of the DC current (the characteristic will be referred to as xe2x80x9cDC superposition characteristicxe2x80x9d or simply as xe2x80x9csuperposition characteristicxe2x80x9d).
As magnetic cores in application fields within high frequency bands, there have been used a ferrite core and a dust core which have individual features due to physical properties of their materials, the ferrite core has a high intrinsic magnetic permeability and a low saturated magnetic flux density while the dust core has a low intrinsic magnetic permeability and a high saturated magnetic flux density. Accordingly, the dust core is often used as one having a toroidal shape. On the other hand, the ferrite magnetic core has an E-shape core part having a central leg formed with a magnetic gap so as to prevent magnetic saturation from being caused by the superposition of DC current.
Recently, since electronic parts are required to be small-sized as electronic devices are more compact-sized, the magnetic core with the magnetic gap is small-sized too. So, there is a strong demand for magnetic cores having an increased magnetic permeability against superposition of DC current.
Generally, it is necessary for the demand to select a magnetic core having a high saturation magnetization, that is, to select a magnetic core that is not magnetically saturated by a high magnetic field applied. The saturation magnetization is inevitably determined by materials and cannot be made as high as desired.
As a solution, it has been conventionally proposed to dispose a permanent magnet in a magnetic gap formed in a magnetic path of a magnetic core, that is, to magnetically bias the magnetic core, to thereby cancel a DC magnetic flux caused by the superposition of DC current.
The magnetic bias by use of the permanent magnet is a good solution to improve the DC superposition characteristic, but it have hardly been brought into a practical use because use of a sintered metallic magnet resulted in considerable increase of a core loss of the magnetic core, while use of a ferrite magnet led in unstable superposition characteristic.
In order to resolve the problems, for example, JP-A 50-133453 discloses to use, as a magnetically biasing magnet, a bond magnet comprising rare-earth magnetic powder with a high magnetic coercive force and binder which are mixed together with each other and compacted into a shape, thereby the DC superposition characteristic and temperature elevation of the core being improved.
Recently, a power supply has been more and more strongly required to improve its power transformation efficiency to such a high level that it is difficult to determine good and bad of magnetic cores for choke coils and transformers by core temperatures measured. Therefore, it is inevitable to determine it from core loss data measured by use of a core-loss measuring device. According to the study by the present inventors, it was confirmed that the core loss has a degraded value in cores having the resistance value disclosed in JP-A 50-133453.
Further, there have recently been demands for coil parts of a surface-mount type. Those coil parts are subjected to reflow soldering process so as to be surface-mounted on a circuit board. It is desired that a magnetic core of the coil part be not degraded in its magnetic properties under conditions of the reflow soldering process. Further, the magnet is desired to have oxidation resistance.
It is a theme of this invention to provide a magnetic core being excellent in magnetic properties and core-loss characteristics and having a magnetically biasing magnet which is disposed in the vicinity of at least one magnetic gap formed in a magnetic path of the core for magnetically bias the core through opposite ends of the magnetic gap.
It is an object of this invention to provide a magnetic core that is excellent in the magnetic properties and core-less characteristics under conditions of the reflow soldering process.
It is another object of this invention to provide an inductance element or part having a magnetic core having excellent DC superposition characteristics and core-loss characteristics.
According to this invention, there is provided a magnetic core having at least one magnetic gap in a magnetic path thereof. The magnetic core comprises a magnetically biasing magnet disposed in the magnetic gap to provide a magnetic bias from opposite ends of the magnetic gap to the core. The magnetically biasing magnet comprises a bond magnet which comprises rare-earth magnetic powder and a binder resin. The rare-earth magnetic powder has an intrinsic coercive force of 5 kOe or more, a Curie temperature Tc of 300xc2x0 C. or more, specific resistance of 0.1 xcexa9xc2x7cm or more, residual magnetization Br of 1000 to 4000 G and coercive force bHc of a B-H curve of 0.9 kOe or more.
It is preferable that the intrinsic coercive force is equal to or larger than 10 kOe, the Curie temperature Tc being equal to or larger than 500xc2x0 C., and the specific resistance being equal to or larger than 1 xcexa9xc2x7cm.
According to another aspect of this invention, there is obtained an inductance part which comprises the magnetic core according to this invention, and at least one winding wound by one or more turns on said magnetic core.